Water comprises around 71% of the earth's surface. That’s common knowledge. Each water molecule is made of 2 hydrogen and 1 oxygen atom. Well, each of those atoms are made of smaller particles known as protons, neutrons, and electrons. What? Middle school chemistry?
But there's more to atoms than just those three particles. Each of those particles is comprised of smaller particles known as quarks.
Now a brief history. The idea of a quark was suggested around the same time by two separate scientists: Murray Gellman and George Zweig in 1964. This was toyed with and tentatively accepted as there was no evidence for their existence for a while.
First there were three proposed quarks hence the name quark; in a certain book Finnegan's Wake written by James Joyce, there is a passage "three quarks for muster mark".
In the same year (1964) Sheldon Glashow and James Bjorken coin the name for the fourth quark (charm).
In 1977, Leon Lederman and crew at Fermilab discover bottom quark, and theorize for its partner, top quark.
In 1995, that theory of top quark is validated when a heavy top quark is found.
Quarks are somewhat unique in that they have fractional charges.
What's a charge?
A charge is a positive, negative or neutral property of an atom, dictated by its reactions when exposed to an electromagnetic field. Note that the charge of 1 assigned to protons is just an arbitrary number created as a point of reference.
WARNING: (Some) Repeated Information Ahead
Well, quarks have fractional charges. Currently there are six known quarks: up, down, strange, charmed, bottom, and top.
Up and down were suggested at the same time, and are named such for their spin.
These are the two naturally occurring quarks. Up quarks have a charge of 2/3rds, and down -1/3rd; and together they compose all of the building blocks of matter.
Protons are built up of two up quarks and one down quark, electrons are three down quarks, and neutrons are an up quark and two down quarks.
This addition and subtraction keeps those arbitrary charge numbers the way they were before, while adding a new concept to physics.
Also part of the first batch of quarks, strange quarks are named after their strange property of surviving much longer than normal quarks.
Charm quarks are named after how charmed the scientists were at how well they fit in to the standard model.
These two quarks have the same charges as up and down: charm with +2/3rds and strange with -1/3rd.
Top and bottom get their names from their charge as well: they mirror up and down quarks with parallel charges. Top has a charge of +2/3rds and bottom -1/3rd.
The top quark is the most massive, and thus, in its short lifespan of 5 * 10^-25 seconds, it can't hadronize, meaning it won't form hadrons (baryons (3 quarks) and mesons (quark and antiquark)).
Bottom quarks are interesting as they are almost always products in top quark decays.
So you could say (as it is said) there are three "generations" of quarks: 1st, 2nd, and 3rd. The latter two all eventually decay into the first.
But what's with these quarks? What is their purpose?
Well, electrons were found to mysteriously bounce off of protons. This was in fact due to the quarks inside of the proton deflecting the electron.
You are probably wondering by now, “How do these quarks stick together? Planets are held together by gravity, molecules by ionic or covalent bonds. So… what about the quarks in atoms?”
Quarks are bound to each other with gluons. Think of it as quarks are glued together.
Actually, scrap that: that’s not how they work. But it’s good to remember it that way. Inside of a hadron, there are gluons as well as quarks. Each quark exchanges its gluons with another quark. This is what causes them to bond together.
So, quarks and gluons are found in atoms. As far as we know, these are the smallest bits of matter out there, but there could be smaller.
Quarks make up everything out there, and though we think we know a lot, we really don’t. For instance, the recently discovered exotic hadrons (2014) break our current rules of quarks and the quark model. But it’s up to us to complete the model for better understanding, and with that we have a lot of work to do.
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